This is about https://gettingstarted.mendixcloud.com/link/module/10/lecture/342 , right?
These instructions can be followed when clicking on 'Navigation' in your Project Explorer.
This is what you will see;
Here you can add 'Navigation Profiles'. change icos and captions and set or unset Role-based home pages.
Do not change the 'System.ShowHomePage' microflow, this microflow is (as you mentioned) called to show either the Default home page or the pages you specified for an User Role (again, screenshot above).
I don't think we are looking at the same thing. This instructions are from section 10.5 about page design and the instructions tell you go into the APP Name properties, not navigation.